Fix issues with JSON and float/numeric

Decimal accuracy and behavior has been improved when extracting floating
point and/or decimal values from JSON strings using the
:meth:`_sql.sqltypes.JSON.Comparator.as_float` method, when the numeric
value inside of the JSON string has many significant digits; previously,
MySQL backends would truncate values with many significant digits and SQL
Server backends would raise an exception due to a DECIMAL cast with
insufficient significant digits.   Both backends now use a FLOAT-compatible
approach that does not hardcode significant digits for floating point
values. For precision numerics, a new method
:meth:`_sql.sqltypes.JSON.Comparator.as_numeric` has been added which
accepts arguments for precision and scale, and will return values as Python
``Decimal`` objects with no floating point conversion assuming the DBAPI
supports it (all but pysqlite).

Fixes: #5788
Change-Id: I6eb51fe172a389548dd6e3c65efec9f1f538012e

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